Trunked RF repeater systems have become a mainstay of modern RF communications systems used, for example, by public service organizations (e.g., governmental entities such as counties, fire departments, police departments, etc.). Such RF repeater systems permit a relatively limited number of RF communications channels to be shared by a large number of users--while providing relative privacy to any particular RF communication (conversation). As is well known, typical state-of-the-art RF repeater systems are "digitally trunked"--e.g., they use digital signals conveyed over the RF channels (in conjunction with digital control elements connected in the system) to accomplish "trunking" (time-sharing) of the limited number of RF channels among a large number of users.
Briefly, such digitally trunked RF communications systems include a "control" RF channel and several (possibly many) "working" RF channels. The working channels are used to carry actual communications traffic (e.g., analog FM, digitized voice, digital data, etc.). The RF control channel is used to carry digital control signals between the repeater sites and user RF transceivers in the field. When a user's transceiver is not actively engaged in a conversation, it monitors the control channel for "outbound" digital control messages directed to it. User depression of a push-to-talk (PTT) switch results in a digital channel request message requesting a working channel (and specifying one or a group of callees) to be transmitted "inbound" over the RF control channel to the repeater site. The repeater site (and associated trunking system) receives and processes the channel request message.
Assuming a working channel is available for use by the requesting user, the repeater site generates and transmits a responsive "outbound" channel assignment digital message over the RF control channel (this message has the effect of temporarily assigning the available working channel for use by the requesting user transceiver and other callee user transceivers specified by the channel request message). The channel assignment message automatically directs the requesting (calling) user transceiver (and callee user transceivers) to the available RF working channel for a communications exchange.
When the communications exchange terminates, the user transceivers "release" the temporarily assigned working channel and return to monitoring the RF control channel. The working channel is thus available for reassignment to the same or different user transceivers via further messages conveyed over the RF control channel.
Single-site trunked RF repeater systems may have an effective coverage area of tens of square miles. It is possible to provide one or more satellite receiving stations (and a single high power transmitting site) if a somewhat larger coverage area is desired. However, some governmental entities and other public service trunking system users may require an RF communications coverage area of hundreds of square miles. In order to provide such very large coverage areas it is necessary to provide multiple RF repeater (transmitting) sites--and to somehow automatically coordinate all such sites so that a user radio transceiver located anywhere in the system coverage area may efficiently communicate in a trunked manner with other user radio transceivers located anywhere in the system coverage area.
FIG. 1 is a schematic diagram of a simplified exemplary multiple-site trunked radio repeater system having three radio repeater (transmitting/receiving) sites S1, S2, and S3 providing communications to geographic areas A1, A2, and A3, respectively. Mobile or portable transceivers within area A1 transmit signals to and receive signals from site S1; transceivers within area A2 transmit signals to and receive signals transmitted by site S2; and transceivers within area A3 transmit signals to and receive signals transmitted by site S3. Each repeater site S1, S2, S3 includes a set of repeating transceivers operating on a control channel and plural RF working channels. Each site may typically have a central site controller (e.g., a digital computer) that acts as a central point for communications in the site, and is capable of functioning relatively autonomously if all participants of a call are located within its associated coverage area.
To enable communications from one area to another, however, some mechanism (e.g., a switching network) must be provided to establish control and audio signal pathways between repeaters of different sites. Moreover, such pathways must be set up at the beginning of each call, and taken down at the end of each call. Unlike the situation in a cellular radiotelephone or land-based telephone environment, trunked radio system calls may be relatively short (e.g., in some cases, as short as a second or two in duration). During an emergency or other period of great activity, the system may be required to handle hundreds of such calls in a very short time period. For this to occur, it is necessary to efficiently and rapidly route audio signals between the elements (i.e., RF transmitter/receiver decks) related to any arbitrary channel of, for example, site S1 and the elements related to any arbitrary channel of site S2 (and also to the elements of any arbitrary channel of site S3) to permit user transceivers in coverage areas A1 and A2 (and A3) to communicate with one another.
Of course, telephone digital switch equipment providing extensive audio routing possibilities has been available for many years. Moreover, trunked radio system manufacturers have also for at least several years offered trunked switches that provided audio (and digital control) signal routing within the context of a trunked radio communications system. As one example, Motorola's Centracom II Plus trunked switch provides digital call routing between RF repeater stations of a single site and associated dispatch consoles. See also prior issued U.S. Pat. No. 4,268,722 which issued on May 19, 1981.
The assignee of the subject application, Ericsson-General Electric (EGE), has for the last few years offered "multi-site" trunked RF communications systems to its customers. Such multi-site systems provide multiple (spatially separated) RF repeater "sites" of the type shown in FIG. 1 in order to cover a large geographical area. A switching network (commonly referred to as a "switch") used by EGE in the past in its multi-site system employed a centralized analog audio routing and mixing matrix manufactured by Console Systems Inc (CSI) in combination with a standard central equipment. telephone switch to provide the audio routing and conferencing capabilities required. This arrangement was found to be too noisy, inflexible, complicated, slow and expensive and also had reliability problems (for example, if the audio mixing matrix failed, the entire switch would cease to function). Moreover, the system was not expandable (e.g., to accommodate additional RF sites and/or channels).
Modern digital switch networks (e.g., of the type used in telephone central equipment and, more recently in cellular radiotelephone switches) typically use a time-division-multiplexed ("TDM") bus as a common "highway" providing multiple time-shared channels for routing digitized audio signals between nodes. To carry the "highway" analogy a bit further, by temporarily creating "entrance/exit ramps" between the TDM bus and nodes that need to have audio signals routed therebetween, the "highway" can be used to carry a high volume of "traffic" between any two (or more) arbitrary nodes. Assume Node A and Node B need to have a bidirectional audio signal pathway temporarily established between them. Node A is assigned a time "slot" (channel) on the TDM bus, and node B is also assigned a (different) TDM time slot (channel). Node A converts the audio signals it needs to communicate to node B into digital (e.g., pulse code modulation--"PCM") signals and places these digital signals into its assigned time "slot" on the bus. Node B similarly converts the audio signals it needs to communicate to node A into digital signals and places these digital signals into its respective TDM bus time slot. To establish a "connection" between nodes A and B, it is only necessary to inform each node which bus "slot" the other node is placing its audio signals into. Each node can then "listen" to the other node's assigned bus slot--providing a bidirectional audio signal pathway between the two nodes.
EGE has also used, in a past multisite system, a TDM-based switch manufactured by CML. The CML switch included multiple TDM busses each operating at 64 KB/s and carrying .mu.law PCM (digitized) audio signals. Audio processors (in the form of cards connected to a backplane) digitized the audio signals received from consoles and radio receivers and provided the digitized signals onto the TDM bus; and also removed digitized audio signals from the bus and converted them back to analog form. The CML system also included a centralized complicated multiple channel audio mixing matrix to route audio signals from any source to any destination as well as to provide conferencing (mixing) capability.
The audio mixing/routing problem is relatively complex. N*N (N.sup.2) possible routing connections are required to ensure that routing can be accomplished between any arbitrary two of N nodes. The problem is still more difficult, however, since the system must be capable of connecting multiple nodes together (e.g., to include multiple call participants located in different repeater coverage areas)--possibly also with one or more dispatch consoles (e.g., so that a dispatcher can listen in on multiple ongoing RF communications exchanges simultaneously and participate in any desired RF communications exchange). The audio routing must also be relatively rapid and have low pending routing request latency time--since calls are typically initiated and terminated very rapidly in a trunked radio repeater system.
Unfortunately, the CML centralized audio routing/mixing matrix described above is expensive, inflexible, unreliable, and relatively incapable of being expanded as the system it serves expands. Due to the nature of a matrix, the size of the matrix increases as the square of the increase in number of input lines to be mixed/routed (making expansion extremely expensive and, beyond a certain point, practically impossible). In addition, a mixing matrix is an extremely complicated piece of equipment that may be subject to reliability problems. Since failure of the matrix causes all audio routing to cease (thus rendering the entire switch non-operational), it was necessary to provide a redundant backup mixing matrix that could automatically be activated if the main matrix failed. Providing such a redundant matrix substantially increased the cost and complexity of the switch.
While prior trunked digital switches did provide audio routing between multiple arbitrary RF repeaters, much further improvement is possible. Lack of reliability and flexibility, high cost and undue complexity have been significant problems in prior art trunked RF switch designs. Moreover, prior art trunked RF switch audio processing circuitry and associated architecture has not been flexible enough to provide all of the features desired in a sophisticated multisite trunked radio system. It would be highly desirable to provide a trunked switch audio processing/routing system and associated architecture that is fault tolerant, can be easily maintained and serviced, is relatively inexpensive, and can be provided in a minimal yet easily (and inexpensively) expandable configuration to provide low latency time, high speed and high traffic carrying capability.
The present invention relates to distributed audio signal routing within a multisite trunked RF switch having a distributed architecture. The logical functions of such a switch are shared by various microprocessor operated nodes distributed throughout the switch. These nodes share the computational workload of the switch. The audio routing provided by the present invention is also performed in a distributed manner by audio processors associated with each audio source/destination--thereby completely eliminating the centralized audio mixing/routing matrix required in prior art trunked RF switches.
The present invention provides a highly advantageous distributed arrangement for routing audio from multiple sources to multiple destinations while at the same time minimizing overhead transactions needed for such audio routing.
In the preferred embodiment, each audio "source" in the multisite system (e.g., each repeater receiver) continually broadcasts audio signals over an audio bus (e.g., a TDM bus). Thus, the audio bus continually carries the audio signals from each of the sources (sufficient bus channel capacity is provided so that all audio sources may broadcast over the bus all the time). The problem of routing temporary audio pathways is thus simplified to selecting a subset of the audio signals carried by the bus to be routed to a particular audio destination (and to perform such selection in parallel in a distributed manner for each audio destination).
In the preferred embodiment, each audio destination continually monitors and processes all audio bus channels (a fast processing arrangement is provided such that the destinations always keep up with the bus traffic). In the preferred embodiment, a fast multiplier/summer is used to multiply each bus audio channel by an associated weighting factor, and to sum all such products together. Thus, selection of bus channels can be accomplished by simply specifying the weighting factors corresponding to the different bus audio channels.
The audio destination arrangement described above operates just as rapidly and efficiently when it is selecting any arbitrary (or all) bus audio channels as it does when it is selecting no bus audio channels. Hence, mixing of audio channels together is accomplished as a matter of course, and any number of bus audio channels can be mixed together efficiently and rapidly.
In somewhat more detail, the preferred embodiment distributed switch provides a digitized audio TDM (time division multiplexed) network that includes plural (e.g., 32) individual TDM busses. Frame and slot timings are synchronized across the busses, and define plural (e.g., 32) time slots per frame. In the presently preferred embodiment, this TDM bus arrangement provides a "highway" carrying many independent digitized audio channels simultaneously.
In the presently preferred exemplary embodiment of the present invention, audio processing circuitry associated with each channel of audio incoming from an RF site is preassigned a TDM bus number and bus slot (such preassignments may be made, for example, at power up time). This "incoming" or source audio processing circuitry continually outputs its digitized audio onto its preassigned TDM bus during its preassigned bus slot. Thus, no ongoing switching or routing of incoming calls onto the audio TDM network is necessary (all incoming calls are always "routed" onto the TDM bus).
Audio signal routing occurs--in a distributed manner--at the "outgoing" (destination) end of the audio connections in the preferred embodiment switch (e.g., at audio processing circuitry coupled to an RF transmitter or console "select" speaker). For example, in the preferred embodiment, audio processing circuitry associated with (and dedicated to) each RF transmitting "channel" is provided. If the associated RF channel is to be involved in a communication, this audio processing circuitry "listens" to selected (specified) digitized audio signals carried on the TDM network, converts those signals to analog audio signals, and provides the analog audio signals to the RF transmitter for transmission over an associated RF link. As part of "setting up" a call for routing through the preferred embodiment switch, a digital message is sent to such destination audio processing circuitry (via an associated controller) specifying which TDM buss(es)/bus slot(s) it is to listen to, convert to analog form, and provide to its associated RF transmitter.
As described above, in the preferred embodiment such audio processing circuitry is capable of mixing together any (or all) of the audio channels carried on the TDM bus. There is thus no need for any centralized audio mixing matrix in the preferred embodiment. Each audio destination on the audio TDM network can mix together audio signals from any/all of the audio sources coupled to the TDM network. Moreover, such mixing is performed in a distributed parallel processing manner for each of the RF channels that are to be involved in the call. Such call routing can be activated extremely rapidly (thus reducing latency time), and is also highly reliable (since failure of an entire audio processor merely results in failure of audio routing with respect to a small number of RF channels).
The distributed audio routing architecture provided by the multisite switch of the present invention thus has several advantages over prior art trunked switch designs. For example, the distributed audio routing arrangement safeguards against catastrophic failures of the switch or of all communications from one RF system to another. Mobile units in the area serviced by a failed node may not be able to call a unit in another area or receive calls from another area, but all other audio routing continues in an unaffected manner.
The distributed network multisite switch provided by the present invention also has a much faster response time than comparable central architecture multisite systems. The distributed network audio routing of the present invention provides parallel processing by sharing the computational task between several processors--and thus offers significant speed increases as compared with prior art systems.
The distributed audio routing arrangement provided by the present invention is also much less costly, much more easily expandable, and less complicated than prior art trunked repeater system centralized audio routing arrangements. The cost of plural individual audio routing processors/modules may be less than the cost of a centralized mixing facility. Moreover, a distributed network switch can be expanded simply by adding further modules. In contrast, to expand the capacity of a centralized mixer may require replacing the entire audio switching network with a larger unit.
The present invention also provides a unique node architecture in which digital trunking signal handling capabilities are integrated with audio signal routing functions. In the preferred embodiment, each node of a multisite network switch is supported by modular switch controllers and associated audio processors. These audio processor modules all have the same architecture and are interchangeable with one another in the preferred embodiment. The same controller modules and audio processor modules can be used in all nodes (e.g., console nodes and RF repeater nodes alike). The multisite node architecture provides for interchangeable node modules that can be used in any node in the switch. The multisite switch can thus be easily serviced in the field by replacing modules. The service person need only stock a few types of modules to replace any node in the switch. The service technician no longer must stock a variety of components to service the switch or review voluminous manuals about the circuitry in each of the various nodes. Similarly, a uniform node architecture reduces the complexity and costs of manufacturing.
The architecture of the node provided by the present invention includes a single controller module (preferably with a backup redundant controller module to ensure functionality in case of failure) supporting a plurality of audio modules. The controller modules and audio modules may take the form of printed circuit boards connected to a common backplane. The audio modules each process audio for several (e.g., four) bidirectional audio channels. Thus, one controller board supports many (e.g., sixteen) audio/data channels. The architecture of the node and its operation are specifically designed to enable a single controller board to handle a large number of audio boards and channels.